Transfer assisted screen printing method of making shaped abrasive particles and the resulting shaped abrasive particles

ABSTRACT

Shaped ceramic articles can be obtained by screen printing the desired shapes from a dispersion of a precursor of the ceramic onto a receiving surface using a transfer assisted technique that applies a differential pressure, at least partially drying the screen printed shapes, and firing them to generate the shaped ceramic articles. Shaped abrasive particles made using lower viscosity sol gels that tended to flow or creep after the screen printing formation were found to have higher grinding performance over screen printed shaped abrasive particles made with higher viscosity sol gels.

This application is a divisional of U.S. application Ser. No.12/827,274, filed Jun. 30, 2010, now allowed, which is a national stagefiling under 35 U.S.C. 371 of PCT/US2010/059830, filed Dec. 10, 2010,which claims priority to U.S. Provisional Application No. 61/289,188,filed Dec. 22, 2009, the disclosures of which are incorporated byreference in their entirety herein.

BACKGROUND

Abrasive particles and abrasive articles made from the abrasiveparticles are useful for abrading, finishing, or grinding a wide varietyof materials and surfaces in the manufacturing of goods. As such, therecontinues to be a need for improving the cost, performance, or life ofthe abrasive particle and/or the abrasive article.

Triangular shaped abrasive particles and abrasive articles using thetriangular shaped abrasive particles are disclosed in U.S. Pat. No.5,201,916 to Berg; U.S. Pat. No. 5,366,523 to Rowenhorst (Re 35,570);and U.S. Pat. No. 5,984,988 to Berg. In one embodiment, the abrasiveparticles' shape comprised an equilateral triangle. Triangular shapedabrasive particles are useful in manufacturing abrasive articles havingenhanced cut rates.

SUMMARY

Shaped abrasive particles, in general, can have superior performanceover randomly crushed abrasive particles. By controlling the shape ofthe abrasive particle, it is possible to control the resultingperformance of the abrasive article. The inventors have discovered thatby making the shaped abrasive particles from a dispersion of an alphaalumina precursor using a screen printing technique with an assistedtransfer of the dispersion from the openings in the printing screen,improved productivity of the process results. Additionally, much thickershaped abrasive particles can be made since it is possible to removethese thicker particles from the screen openings.

The assisted transfer can comprise applying a differential pressurebetween a first side of the screen printed shape and a second side ofthe screen printed shape that is in contact with a receiving surface. Inone embodiment, a pressurized transfer roll can apply a positivepressure to the first side to release the screen printed shapes from theprinting screen in the form of a continuous printing belt. In anotherembodiment, the receiving surface can comprise an air permeablereceiving surface and a vacuum box or a vacuum roll can be located toprovide a pressure less than atmospheric pressure to the second side ofthe screen printed shapes as they are removed from the apertures in theprinting screen. Alternatively, a positive pressure can be applied tothe first side of the screen printed shapes while simultaneouslyapplying a pressure lower than atmospheric pressure to the second side.

Briefly, in one embodiment, the method encompasses the production ofshaped abrasive particles by a screen printing process which comprisesapplying a dispersion to a receiving surface through a printing screenhaving a plurality of apertures corresponding to the desired shape ofthe shaped abrasive particles. A preferred process comprises applying adispersion of an alpha alumina precursor to one surface of an aperturedprinting screen supported on a receiving surface, filling the aperturesin the printing screen, removing the printing screen from the receivingsurface while applying a differential pressure between a first side of ascreen printed shape and a second side of a screen printed shape incontact with the receiving surface, at least partially drying thedispersion to form precursor shaped abrasive particles and firing theprecursor shaped abrasive particles to final sintered hardness formingshaped abrasive particles comprising alpha alumina.

Surprisingly, the inventors have determined that a sol gel dispersionhaving a lower viscosity that creeps or flows under its own weight afterthe formation process provides shaped abrasive particles having improvedgrinding performance over shaped abrasive particles made by the sameprocess using a higher viscosity dispersion. Thus, perfectlydimensionally stable sol gel dispersions are not preferred in someembodiments and shaped abrasive particles having a larger second sidethan the first side and a slumping sidewall have been unexpectedly foundto have better grinding performance. The resulting shaped abrasiveparticle tapers from the larger second side (the side in contact withthe receiving surface during drying) to the smaller first side(unsupported air side) even though the apertures used to made the shapedabrasive particle were not tapered (approximately 90 degree sidewall).

The tapering is believed to occur during the drying process or,alternatively, or in combination as a result of the differentialpressure applied when removing the screen printed shapes with lowerviscosity sol gels. It is possible that the lower viscosity sol gelscreep or flow under their own weight during the drying process beforebecoming substantially dry similar to the way cookie dough in an oven istransformed from an initial round ball into its final disk shape. Thus,the drying process produces shaped abrasive particles having thinneredges with an acute angle between the sidewall of the shaped abrasiveparticle and the second side. This thinner edge is believed to enhancethe grinding performance.

Hence in one embodiment, the invention resides in a process for theproduction of shaped ceramic articles by a screen printing process whichcomprises applying a dispersion of a ceramic precursor to a receivingsurface through a printing screen comprising a plurality of apertures,removing the printing screen from the receiving surface to form aplurality of screen printed shapes while applying a differentialpressure between a first side of the screen printed shape and a secondside of the screen printed shape that is in contact with the receivingsurface, at least partially drying the screen printed shapes remainingon the receiving surface and firing the screen printed shapes to formsintered shaped ceramic articles.

In another embodiment, the invention resides in a process for theproduction of shaped abrasive particles comprising alpha alumina, theprocess comprising screen printing shapes of a boehmite alumina sol gelonto a receiving surface to form a plurality of screen printed shapes,removing the screen printed shapes from a printing screen while applyinga differential pressure between a first side of the screen printedshapes and a second side of the screen printed shapes that are incontact with the receiving surface, at least partially drying the screenprinted shapes and firing the screen printed shapes at a temperaturesufficient to convert the alumina to the alpha phase.

In another embodiment, the invention resides in a process for theproduction of shaped abrasive particles comprising alpha alumina, theprocess comprising screen printing shapes of a boehmite alumina sol gelonto an air permeable receiving surface, the air permeable receivingsurface located adjacent to a vacuum box or vacuum roll while fillingapertures in a printing screen with the sol gel to form a plurality ofscreen printed shapes, removing the screen printed shapes from theprinting screen, at least partially drying the screen printed shapes,and firing the screen printed shapes at a temperature sufficient toconvert the alumina to the alpha phase.

In another embodiment, the invention resides in shaped abrasiveparticles comprising a post-formation flowed surface.

In another embodiment, the invention resides in shaped abrasiveparticles comprising a first side, a second side, and a sidewallconnecting the first side to the second side; the sidewall comprising afirst portion intersecting the first surface and a second portionintersecting the second surface and the slope of the first portion isgreater than the slope of the second portion.

In another embodiment, the invention resides in shaped abrasiveparticles comprising a first side, a second side, and a sidewallconnecting the first side to the second side; and wherein the first sideis concave and the sidewall is concave.

BRIEF DESCRIPTION OF THE DRAWINGS

It is to be understood by one of ordinary skill in the art that thepresent discussion is a description of exemplary embodiments only, andis not intended as limiting the broader aspects of the presentdisclosure, which broader aspects are embodied in the exemplaryconstruction.

Repeated use of reference characters in the specification and drawingsis intended to represent the same or analogous features or elements ofthe disclosure.

FIG. 1 is schematic representation of a screen printing processaccording to the invention for continuous mode operation.

FIG. 2 is a photograph of the top surface of a printing screen suitablefor use with either the continuous mode or a batch mode screen printingprocess according to the invention.

FIGS. 3A, 3B, and 3C are photomicrographs showing top, side andperspective views of shaped abrasive particles made with a higherviscosity sol gel using the printing screen illustrated in FIG. 2.

FIGS. 4A, 4B, and 4C are photomicrographs showing top, side, andperspective views of shaped abrasive particles made with a lowerviscosity sol gel using the printing screen illustrated in FIG. 2.

FIG. 5 is a graph of cut versus time comparing the grinding performanceof shaped abrasive particles made with the high and low viscosity solgels.

DEFINITIONS

As used herein, “generally triangular” means a triangular shape orthree-sided tapered or non-tapered polygons having rounded cornersinstead of vertices as shown in FIGS. 2, 3, and 4.

As used herein, a “post-formation flowed surface” means a surface of ashaped abrasive particle that significantly slumps after removal of theshaped dispersion from the printing screen or mold. As such, the shapedabrasive particle will have a larger surface area on one side due to theslumping and a thickness or height which is less than a shaped abrasiveparticle made from the same printing screen or mold that does not slumpsignificantly. A post-formation flowed surface is a relatively smoothsurface having smooth contours and slopes resulting from relocation ofthe shaped dispersion due to forces (such as gravity or a differentialapplied pressure) under conditions (such as lower viscosity sol gels orslower drying rates) that allow the slumping to occur. Flashing orextremely thin areas of sol gel attached to the surfaces of the shapedabrasive particle due to overfilling of the printing screen or mold suchthat sol gel extends past the intended predetermined geometric shape inthe printing screen or mold is not a post-formation flowed surface.

As used herein, a “shaped abrasive particle” means an abrasive particleformed by placing a dispersion into apertures in a printing screen orinto a mold cavity, the apertures or the mold cavities having apredetermined geometric shape that produces abrasive particles whichresemble that shape, but may not perfectly replicate it.

As used herein, a “textured surface” means a shaped abrasive particlesurface having a rough or grainy surface quality. A textured surface canbe produced by an air permeable receiving surface in combination withvacuum that pulls the sol gel into intimate contact with the receivingsurface. The textured surface may partially replicate the weave or meshof the receiving surface, the structure of entangled fibers forming thereceiving surface, or the outlines of apertures in the receivingsurface.

DETAILED DESCRIPTION

A printing screen 10 in the form of a continuous printing belt passesaround a series of three rolls 12, 14, and 16. In one embodiment, nearthe point of separation of the printing screen 10 from a receivingsurface 18, a pressure transfer roll 20 is located. In anotherembodiment, a conventional roll can be substituted for the pressuretransfer roll 20. The space between roll 14 and the pressure transferroll 20 defines an application zone 22; the area including the pressuretransfer roll 20 and the open space between the pressure transfer rolland roll 16 defines a disengagement zone 24; the space between rolls 16and 12 defines a cleaning zone 26; and the space between rolls 12 and 14defines a treatment zone 30.

In the application zone 22, the printing screen 10 is held in firmcontact with a continuous belt 32 along its outside surface while bothbelts move in the same direction at essentially the same speed and adispersion of ceramic precursor particles is applied to the insidesurface of the printing screen 10, (application mechanism is not shown)ahead of a doctor blade 34. A suitable pump and a manifold or slotteddie can be used to apply the dispersion to the printing screen. Thepassage beneath the doctor blade forces the dispersion into apertures inthe printing screen 10 which, at that point, is in firm contact with thecontinuous belt 32. If desired, a support roll can be located underneaththe continuous belt 32 opposite the doctor blade 34 to reduce deflectionof the printing screen 10 and the continuous belt 32. In anotherembodiment, a vacuum box 46 or vacuum roll can be located on theopposite surface of the continuous belt 32 having an air permeablestructure. The source of vacuum can be used to assist in filling theapertures with the dispersion; especially, for high viscositydispersions.

In the disengagement zone 24, the printing screen 10 is disengaged fromthe continuous belt 32 leaving one or more screen printed shapes 36 onthe receiving surface 18. The disengagement zone 24 is provided with anassisted transfer of the dispersion from the apertures in the printingscreen 10. The assisted transfer can comprise applying a differentialpressure between a first side 38 and a second side 40 of the screenprinted shape 36 that is in contact with the receiving surface 18 whilebeing removed from the aperture in the printing screen 10.

In one embodiment, the pressure transfer roll 20 can apply a positivepressure to the first side 38 to push the screen printed shape 36 fromthe aperture in the printing screen 10. The pressure transfer roll cancomprise a hollow roll with a drilled outer shell. The outer shell canhave a mesh, screen, fabric, or nonwoven applied over it to furtherdistribute the pressurized fluid supplied to the pressure transfer roll20 after passing through holes in the outer shell. Internally, fixed oradjustable baffles can be supplied to create a radial pressure zone 42that concentrates the pressurized fluid supplied to the pressuretransfer roll. The width and angular location of the radial pressurezone along with the operating pressure of the pressurized fluid and themachine direction location of the pressure transfer roll can be adjustedto cleanly remove the dispersion from the apertures in the printingscreen without unduly distorting the screen printed shapes. Pressuretransfer rolls (or adjustable vacuum rolls) having these and otherfeatures are known in the art of paper manufacturing equipment. In oneembodiment, the pressurized fluid was air supplied by a compressor 44and fluidly connected through a rotary union and piping to the radialpressure zone 42.

In another embodiment, the receiving surface 18 can comprise an airpermeable receiving surface (for example a mesh or porous belt or fabricsuch as those used in the dewatering of paper during its manufacture)and a vacuum box 46 or a vacuum roll can be located in the disengagementzone 24 to provide less than atmospheric pressure to the second side 40of the screen printed shape 36 to pull the dispersion from the aperturein the printing screen 10. The vacuum box 46 or vacuum roll is fluidlyconnected to a vacuum pump 48. Suitable vacuum boxes or vacuum rolls areknown in the paper making art. Additionally, the vacuum box can extendinto the application zone 22 or additional vacuum boxes 46 or vacuumrolls can be placed in the application zone 22 to assist with fillingthe apertures in the printing screen. Suitable control valves 47 can beused to regulate the vacuum applied to each zone.

Suitable air permeable receiving surfaces include any porous materialhaving pores significantly smaller than the apertures in the printingscreen. As mentioned, woven fabrics and felts as known in the paperindustry can be used. Nonwoven fabrics can be used. Corrosion resistantmetal screens or metal meshes can be used. Thin metal bands or beltswhich have been machined, stamped, or altered to render them airpermeable can used. Some materials may require a suitable release agentto be applied such as oils or a coating of polytetrafluoroethene.Acceptable materials include stainless steel and polyethylene.

In one embodiment, a solid roll positioned in place of the pressuretransfer roll 20 can be used in combination with the vacuum box 46 andthe air permeable receiving surface. The machine direction width of thevacuum zone, the machine direction location of the vacuum box 46 orvacuum roll, and the magnitude of the vacuum supplied can be adjusted tocleanly remove the dispersion from the apertures in the printing screenwithout unduly distorting the screen printed shape.

Alternatively, in another embodiment, a pressure higher than atmosphericpressure can be applied to the first side 38 of the screen printed shape36 while also applying a pressure lower than atmospheric pressure to thesecond side 40 by using both the pressure transfer roll 20 and thevacuum box 46 as illustrated in FIG. 1. As discussed above, theoperating parameters for the pressure transfer roll and the vacuum boxcan be adjusted to cleanly remove the screen printed shape withoutunduly distorting the screen printed shape. In some embodiments, thepressure transfer roll 20 and the vacuum box 46 may be staggered in themachine direction instead of directly opposed to each other and eitherapparatus may be located ahead of the other apparatus relative to thedirection of travel of the continuous belt 32.

The inventors have determined that using an assisted transfer isnecessary to make screen printed shapes as the thickness of the screenprinted shape is increased. When the thickness of the printing screen isincreased beyond about 0.010″ (0.25 mm) some sol gels typically used toform ceramic abrasive particles release poorly from the apertures in theprinting screen. The inventors have produced screen printed shapes usinga printing screen having a thickness of 0.030″ (0.76 mm) on a breathablerelease liner (Part-Wick #4400 manufactured by US Paper Corporation)which was positioned over a vacuum box on a plastic support grid at avacuum of 4.5 inches of water prior to filling the apertures in theprinting screen with the sol gel. If the vacuum was turned off, therewas poor release and extensive deformation of the screen printed shapeswhen trying to remove the printing screen with a significant amount ofthe sol gel remaining in the apertures of the printing screen afterremoval.

After removal of the printing screen 10, the screen printed shapes 36are transported by the continuous belt 32 to a drying zone 50 wheremoisture is withdrawn from the screen printed shapes, at least to theextent necessary to convert them to precursor abrasive particles 52which retain their structural integrity upon handling. The precursorabrasive particles 52 are then removed from the continuous belt 32 andfired in a suitable furnace or kiln to convert them to shaped abrasiveparticles 55. Before the continuous belt 32 enters the application zone22 in contact with the printing screen 10 it may be given a releasetreatment (such a fluorocarbon spray) if the continuous belt has notbeen pre-treated to give it a baked-on release layer.

Meanwhile, the printing screen 10 after leaving the disengagement zone24 passes through the cleaning zone 26 in which the printing screen iscleaned, if necessary, to remove any residual dispersion remaining onthe printing screen 10 by suitable high pressure liquid sprays, liquidbaths, brushes, air blasts or combinations thereof, and then dried ifneeded. From the cleaning zone 26, the printing screen 10 passes to thetreatment zone 30 in which a release agent may, if desired, be appliedto enhance separation of the screen printed shapes 36 from apertures inthe printing screen in the disengagement zone 24.

There are many variations that may be made in the arrangement describedin the drawings. For example, the application of the dispersion to theprinting screen can be made as the belt passes vertically as opposed tohorizontally. The printing screen 10 may also be provided by the surfaceof a single hollow drum with the zones 22, 24, 26, and 30 provided bydifferent segments of the circumference of the drum. All such variationsare embraced within the scope of this invention.

The shape of the apertures may be varied and selected according to thefinal use of the shaped particles. It will however be clear that theprocess offers a method of producing virtually identical shapes in largequantities or, if desired, an exact mix of a variety of pre-determinedshapes. The preferred application of the process of the invention is theproduction of ceramic shaped abrasive particles; although it is usefulto produce ceramic shapes not intended for use in an abrasive article.These shapes may be angular or round and useful shapes include regularrectangular shapes with an aspect ratio, that is the ratio of length tothe greatest cross-sectional dimension, of from about 2:1 to about 50:1and preferably from about 5:1 to about 25:1. Similar aspect ratios canbe used for shapes other than rectangular ones.

Other useful shapes include shaped abrasive particles comprising thindisks having various geometric shapes such as triangles, squares,rectangles, or circles as disclosed in U.S. Pat. No. 5,201,916 to Berg;U.S. Pat. No. 5,366,523 to Rowenhorst (Re 35,570); and U.S. Pat. No.5,984,988 to Berg. A particularly preferred shape is an equilateraltriangular shape as shown in FIGS. 3 and 4. As will be discussed later,using a lower viscosity sol gel such that second side of the shapedabrasive particle is lager than the first side and the edges of theshaped abrasive particle are slumped have been found to have enhancedgrinding performance.

The apertured screen can be made from any suitable material such asstainless steel, plastic such as PTFE, EVA, polyester or nylon, fabricsand the like. The thickness of the printing screen can vary and may bechanged to vary the thickness of the resulting screen printed shape.Suitable thicknesses for the printing screen are typically about 10 mmand more typically 3 mm or less. Screen printing is a well-knownprocedure and materials generally suitable for producing the screens aregenerally also useful in this invention. Suitable release agents such assilicones, fluorocarbons or hydrocarbon derivatives can be applied inthe treatment zone 30, if desired, to improve the release properties ofthe dispersion from the screen. Similar release agents can be used onthe continuous belt 32 or on the air permeable receiving surface.

The apertured printing screen used in the screen printing operation canbe readily adapted for use in a batch mode or in a continuous mode. Whenthe screen thickness is larger (around 10 mm), the screen is generallynot flexible enough to use a continuous process in which the printingscreen is in the form of a continuous printing belt. For mostapplications, the preferred operation is in a continuous productionmode. In such an automated operation, the apertured printing screenusually takes the form of a driven printing belt and this implies thatthe predominant stress on the printing belt is in the longitudinaldirection, that is, it will tend to be stretched. If the largestdimension of the apertures is aligned in the direction of movement ofthe printing belt, the tendency to stretch will lead to less distortionof the cross-section of the apertures. This is a desired orientation ofthe apertures when using a printing screen in the form of a drivenprinting belt.

When in use the printing screen 10, when in the form of a printing belt,is in contact with the continuous belt 32 as it passes through theapplication zone 22. The printing belt therefore should preferably bemade from a moisture resistant material to ensure it is not affected bythe water or acid content of the sol gel. Since the printing belt isalso driven, it is preferred that it be relatively inextensible. Itshould preferably also be substantially smooth so as to avoid the solgel penetrating the material of the printing belt making separationdifficult. Many alumina sol gels have an acidic pH, especially if theyhave been peptized by addition of an acid, and therefore the preferredprinting belts should have substantial corrosion resistance. Preferredmaterials meeting these criteria include stainless steel, chrome-platednickel, polytetrafluoroethylene, copolymers comprising a fluorinatedethylene monomer component, and polypropylene. These materials are alsosuitable to make the continuous belt 32 having an air permeablereceiving surface.

For some materials used to make the printing screen, it is desirable tocoat the material with a release coating to facilitate removal of thescreen printed shapes from the printing screen. The release coating can,for example, be a baked-on fluoropolymer such as that commonly soldunder the DuPont Co. trademark “Teflon”. Alternatively the coating canbe sprayed on before use. Such spray on coatings would include organiclubricants such as octanol, decane, hexadecane, oils, and the like. Itis understood the same considerations apply when designing the aperturedscreen in a different form such as a simple sheet as might beappropriate for operation in a batch mode. These materials are alsosuitable for use as release agents on the continuous belt 32 having anair permeable receiving surface.

FIG. 2 is a photomicrograph showing a top view of one embodiment of aprinting screen. The printing screen was constructed from apolycarbonate sheet that was 0.030″ (0.76 mm) thick. The apertures inthe printing screen formed a pattern of nested rows of equilateraltriangles that were 0.110″ (2.79 mm) per side and the rows were 0.100″(2.54 mm) apart at the apexes or bases. The apertures were made usinglaser machining techniques and the apertures were not tapered such thatthe openings in the top and bottom surfaces of the printing screen wereapproximately the same size.

Materials that can be made into shaped ceramic objects using the processof the invention include physical precursors such as finely dividedparticles of known ceramic materials such as alpha alumina, siliconcarbide, alumina/zirconia and CBN. Also included are chemical and/ormorphological precursors such as aluminum trihydrate, boehmite, gammaalumina and other transitional aluminas and bauxite. The most useful ofthe above are typically based on alumina and its physical or chemicalprecursors. It is to be understood however that the invention is not solimited but is capable of being adapted for use with a plurality ofdifferent precursor materials.

Other components that have been found to be desirable in certaincircumstances for the production of alumina-based particles includenucleating agents such as finely divided alpha alumina, ferric oxide,chromium oxide and other materials capable of nucleating thetransformation of precursor forms to the alpha alumina form; magnesia;titania; zirconia; yttria; and rare earth metal oxides. Such additivesoften act as crystal growth limiters or boundary phase modifiers. Theamount of such additives in the precursor is usually less than about 10%and often less than 5% by weight (solids basis).

It is also possible to use, instead of a chemical or morphologicalprecursor of alpha alumina, a slip of finely divided alpha aluminaitself together with an organic compound that will maintain it insuspension and act as a temporary binder while the particle is beingfired to essentially full densification. In such cases, it is oftenpossible to include in the suspension materials that will form aseparate phase upon firing or that can act as an aid in maintaining thestructural integrity of the shaped particles either during drying andfiring, or after firing. Such materials may be present as impurities.If, for example, the precursor is finely divided bauxite, there will bea small proportion of vitreous material present that will form a secondphase after the powder grains are sintered together to form the shapedparticle.

The dispersion used is able to be formed into a screen printed shapethat resembles the aperture in the screen; although, as discussed later,it may be desirable for this shape to change by fluid flow or solidsmechanics after the printing process becoming larger at the base andhaving slumping sidewalls. Necessary stability could be achieved by theuse of additives where a gel of the precursor is not readily obtained.The solids content of the dispersion is a factor in the invention ifshape retention is not achieved by the use of additives. At the lowersolids contents it may be necessary to adjust the viscosity to preventtotal loss of the printed shapes when the printing screen is removed.This may be done by addition of a viscosity adjustment agent. Where thedispersion is a sol gel the preferred technique is peptization using anacid such as nitric acid. If the dispersion has too high a solidscontent however, there may be difficulty in filling the screen aperturesconsistently. A vacuum box 46 or vacuum roll located in the applicationzone 24 adjacent to the air permeable receiving surface opposite theassembly filling the apertures in the printing screen with thedispersion can be used to overcome or reduce such inconsistencies.

The dispersion that is employed in the process of the invention may beany dispersion of a ceramic precursor such as a finely dispersedmaterial that, after being subjected to the process of the invention, isin the form of a shaped ceramic article. The dispersion may bechemically a precursor, as for example boehmite is a chemical precursorof alpha alumina; a morphological precursor as for example gamma aluminais a morphological precursor of alpha alumina; as well as (oralternatively), physically a precursor in the sense of that a finelydivided form of alpha alumina can be formed into a shape and sintered toretain that shape.

Where the dispersion comprises a physical or morphological precursor asthe term is used herein, the precursor is in the form of finely dividedpowder grains that, when sintered together, form a ceramic article, suchas an abrasive particle of utility in conventional bonded and coatedabrasive applications. Such materials generally comprise powder grainswith an average size of less than about 20 microns, preferably less thanabout 10 microns and most preferably less than about a micron.

The dispersion used in a preferred process is most conveniently aboehmite sol gel. The sol gel may be a seeded sol gel that comprisesfinely dispersed seed particles capable of nucleating the conversion ofalumina precursors to alpha alumina or an unseeded sol gel thattransforms into alpha alumina when sintered.

The solids content of the dispersion of a physical or a morphologicalprecursor is preferably from about 40 to 65% though higher solidscontents of up to about 80% can be used. An organic compound isfrequently used along with the finely divided grains in such dispersionsas a suspending agent or perhaps as a temporary binder until the formedparticle has been dried sufficiently to maintain its shape. This can beany of those generally known for such purposes such as polyethyleneglycol, sorbitan esters and the like.

The solids content of a precursor that changes to the final stableceramic form upon heating may need to take into account water that maybe liberated from the precursor during drying and firing to sinter theabrasive particles. In such cases the solids content is typicallysomewhat lower such as about 75% or lower and more preferably betweenabout 30% and about 50%. With a boehmite sol gel, a maximum solidscontent of about 60% or even 40% is preferred and a sol gel with apeptized minimum solids content of about 20% may also be used.

Abrasive particles made from physical precursors will typically need tobe fired at higher temperatures than those formed from a seeded chemicalprecursor. For example, whereas particles of a seeded boehmite sol gelform an essentially fully densified alpha alumina at temperatures belowabout 1250 degrees C., particles made from unseeded boehmite sol gelsmay require a firing temperature of above about 1400 degrees C. for fulldensification.

Shaped abrasive particles that have been fired to final hardness may beincorporated into a bonded abrasive article such as a grinding wheel, orused to make a coated abrasive article such as a grinding disc or belt,or incorporated into a nonwoven abrasive article, or incorporated intoan abrasive brush. Alternatively, the abrasive grain can be screened orgraded to an abrasive industry specified grade for sale or use in theabove mentioned products. Abrasive industry specified grades couldinclude ANSI standards, FEPA standards, JIS standards, or shapedabrasive particles graded to a nominal screen grade using ASTM StandardTest Sieves (for example—18+20 meaning the shaped abrasive particlespass through a number 18 sieve and are retained on a number 20 sieve).

Referring now to FIGS. 3 and 4, and as further discussed in theExamples, shaped abrasive particles were made using the printing screenof FIG. 2 in a batch mode. The viscosity of the sol gel was adjusted tomade shaped abrasive particles using both a higher viscosity sol gelhaving a higher maximum yield stress and a lower viscosity sol gelhaving a lower maximum yield stress from the same printing screen.

As seen in FIG. 3A, the shaped abrasive particles made with the higherviscosity sol gel having a maximum yield stress of 80,154 Pa had muchless taper from the second side 40 to the first side 38. The perimeterof the second side 40 is slightly larger than the perimeter of the firstside 38 and less of the sidewall 54 connecting the first side to thesecond side is visible in the photomicrograph. The first side 38 or theair side of the shaped abrasive particle during drying is seen outlinedin the center of each shaped abrasive particle in FIG. 3A. The secondside 40 or the supported side of the shaped abrasive particle in contactwith the receiving surface 18 during dying is partially visible in FIG.3B. The second side 40 comprises a textured surface produced by thevacuum pulling the sol gel at least partially into the air permeablereceiving surface. The sidewall 54 connecting the first side 38 to thesecond side 40 as seen in FIG. 3B is scored or ridged tending toreplicate the ridges in the printing screen from the laser machining asseen in FIG. 2. Additionally, the sidewall 54 has a fairly steep slopehaving approximately the same gradient at all locations between thefirst side 38 and the second side 40. The shaped abrasive particlesafter drying tend to have a concave first side 38 and a slightly convexor flat second side 40 as seen in FIG. 3C. The shaped abrasive particlestaper slightly from the second side 40 to the first side 38; however,the angle or slope of the sidewall is quite large and much greater than60 degrees relative to the second side 40. It is estimated that thesidewall angle relative to the second side 40 is between about 70 toabut 80 degrees based on the photomicrographs.

As seen in FIG. 4A, the shaped abrasive particles made with the lowerviscosity gel having a maximum yield stress of 10,154 Pa had much moretaper from the second side 40 to the first side 38. The perimeter of thesecond side 40 is significantly larger than the first side 38 and moreof the sidewall 54 connecting the first side 38 to the second side 40 isvisible in the photomicrograph. The sidewall 54 comprises apost-formation flowed surface. As such, the shaped abrasive particlestend to be thinner than the shaped abrasive particles produced from thehigh viscosity sol gel, which is seen by comparing FIG. 3B to FIG. 4B.The first side 38 or the air side of the shaped abrasive particle duringdrying is seen outlined in the center of each shaped abrasive particlein FIG. 4A. The second side 40 or the supported side of the shapedabrasive particle in contact with the receiving surface 18 during dyingis partially visible in FIG. 4B. The second side 40 comprises a texturedsurface produced by the vacuum pulling the sol gel at least partiallyinto the air permeable receiving surface. The sidewall 54 connecting thefirst side 38 to the second side 40 as seen in FIG. 4B is not scored orridged and is fairly smooth due to the sol gel flow or creep afterremoval of the screen printed shapes from the printing screen. Theridges visible in the sidewall 54 of the shaped abrasive particles inFIG. 3B are no longer visible in FIG. 4B even though the same printingscreen was used.

Additionally, the sidewall 54 does not have a substantially uniformslope or gradient along its length. As best seen in FIG. 4C, thesidewall 54 of the shaped abrasive particle 55 comprises a first portion56 intersecting with the first side 38 having a steeper slope or anglethan a second portion 58 intersecting with the second side 40.Additionally, the length of the first portion 56 is shorter than thelength of the second portion 58. Although such a sidewall configurationwas produced by slumping of the sol gel after being screen printed it ispossible to design a printing screen or a mold having one or both ofthese features to mold the sidewall 54 into this configuration whenusing high viscosity sol gels. The sidewall 54 could comprise a firstsurface 56 intersecting a second surface 58 at a predetermined angle andthe length of each surface could be varied as desired with eithersurface being larger than the other.

The shaped abrasive particles after drying tend to have a concave firstside 38 and a slightly convex or flat second side 40 as seen in FIG. 4B.The shaped abrasive particles taper significantly from the second side40 to the first side 38 and the angle or slope of the sidewall at whichthe sidewall 54 intersects with the second side 40 is much smaller. Theangle, as best seen in FIG. 4B,was measured for 20 shaped abrasiveparticles using image analysis techniques and the average angle wasfound to be 33.9 degrees, which is much less than the expected angle of90 degrees since the apertures in the printing screen were not tapered.

In various embodiments of the invention, the angle between the sidewall54 and, specifically, the second portion 58 and the second side 40 canbe less than about 60, 50, 45, or 40, or 35 degrees and greater thanabout 10 degrees. In general, if angle is less than about 10 degrees theedges of the shaped abrasive particle can become too weak and fracturetoo readily during use for some applications.

Physical Testing

Maximum Yield Stress

The sol gel maximum yield stress (maximum viscosity as a function ofshear rate) is measured using a rheometer such as a Bohlin Gemini 200available from Malvern instruments Ltd, having an office inWorcestershire, United Kingdom. The measurement of the sol gel is madeafter coming to temperature equilibrium at 25 degrees Celsius at shearrates between 0.01 and 1000 sec−1. In various embodiments of theinvention, the maximum yield stress of the sol gel to produce screenprinted shapes that creep or flow during drying to produce the sinteredshaped abrasive particles can be less than about 90,000; about 80,000;about 70,000; about 60,000; about 50,000; about 40,000; about 30,000;about 20,000; or about 15,000 Pa·Sec [Pascal(seconds)]; and greater thanabout 5,000 Pa·Sec. If the viscosity becomes too low, the screen printedshape will no longer resemble the predetermined shape formed in theprinting screen or mold after removal of the dispersion from theprinting screen or mold.

Second Side to First Side Area Ratio

The Second Side to First Side Area Ratio is determined by image analysison the densified, sintered shaped abrasive particle. The shaped abrasiveparticle is mounted by its second side 40 and positioned in a scanningelectron microscope (SEM) to take a top view photomicrograph of thefirst side 38 (SEM line of sight at approximately 90 degrees to thefirst side) using back scattered electron imaging in an SEM. Anappropriate magnification is used such that the entire shaped abrasiveparticle can be viewed, and 1 to 2 complete shaped abrasive particlesfill the SEM's field of view. A typical SEM image of the shaped abrasiveparticles is shown in FIGS. 3A and 4A at 50× magnification.

Next, using image analysis software such as Image J (available from theNational Institute of Health), the area of the first side 38 bounded byits perimeter and the area of the second side 40 bounded by itsperimeter is measured for 10 different shaped abrasive particles. Insome instances, it may be necessary to trace the perimeter manually ifsufficient contrast is not present for the image analysis software toautomatically detect each perimeter. The ten first side areas and theten second side areas are individually averaged and the Second Side toFirst Side Area Ratio is determined by the Average Second Side Areadivided by the Average First Side Area. A larger Second Side to FirstSide Area Ratio results from increased sol gel slumping when the top andbottom openings in the printing screen are of the same size or when theprinting screen or mold is intentionally tapered. A ratio of 1.0 wouldoccur for perfectly dimensionally stable dispersions after screenprinting with a non-tapered aperture. The shaped abrasive particles ofFIG. 3 were found to have a Second Side to First Side Area Ratio of 1.65with a standard deviation of approximately 0.15 if 10 individual ratioswere calculated instead of averaging the 10 separately measured firstside and second side areas. The shaped abrasive particles of FIG. 4 werefound to have a Second Side to First Side Area Ratio of 3.44 with astandard deviation of approximately 0.44 if 10 individual ratios werecalculated instead of averaging the 10 separately measured first sideand second side areas.

In various embodiments of the invention, the Second Side to First SideArea Ratio can between about 1.5 to about 10.0, or between about 2.0 toabout 10.0, or between about 2.0 to about 6.0, or between about 3.0 toabout 5.0, or between about 2.0 to about 3.5, or between about 1.5 toabout 3.5.

Thickness Ratio

To calculate the thickness ratio, fifteen randomly selected shapedabrasive particles are screened. The height of each corner of eachparticle is measured and then all of the heights are averaged todetermine an average Tc. For example, a triangle would have three Tcmeasurements per shaped abrasive particle and 45 measurements total foruse in determining the average for Tc. If the shaped abrasive particleis round, oval or otherwise does not have corners or points, then threepoints equidistant from each other along the perimeter should bemeasured for each shaped abrasive particle. Next, the smallestthickness, Ti, for the interior of the first side 38 of each shapedabrasive particle is measured. Often the translucency of the shapedabrasive particle can be used to find the minimum interior thickness andthe 15 results are averaged to determine an average Ti. The thicknessratio is determined by dividing the average Tc by the average Ti.

A light microscope equipped with an X-Y stage and a vertical locationmeasurement stage can be used to measure the thickness of variousportions of the shaped abrasive particles. Triangular shaped particlesproduced by the prior art method disclosed in U.S. Pat. No. 5,366,523entitled Abrasive Article Containing Shaped Abrasive Particles toRowenhorst et al. have been measured to have thickness ratios between0.94 to 1.15, meaning they are essentially flat and are just as likelyto be slightly thicker in the middle as they are to be slightly thinnerin the middle. Shaped abrasive particles having a thickness ratiogreater than 1.20 are statistically different from the Rowenhorstparticles at the 95% confidence interval. Shaped abrasive particleshaving a recessed surface are also disclosed in U.S. patent applicationSer. No. 12/336,961 filed on Dec. 17, 2008 entitled Dish-Shaped AbrasiveParticles With A Recessed Surface. In various embodiments of theinvention, a Thickness Ratio of Tc/Ti of the concave first side 38 canbe between about 1.20 to about 5.00, or between about 1.25 to about5.00, or between about 1.25 to about 4.00, or between about 1.25 toabout 2.00.

EXAMPLES

Objects and advantages of this disclosure are further illustrated by thefollowing non-limiting examples. The particular materials and amountsthereof recited in these examples as well as other conditions anddetails, should not be construed to unduly limit this disclosure. Unlessotherwise noted, all parts, percentages, ratios, etc. in the Examplesand the rest of the specification are by weight.

Gel Preparation

Boehmite gels (Gel A and Gel B) of two different viscosities were madeby dispersing aluminum oxide monohydrate powder having the tradedesignation “DISPERAL” (Sasol North America, Houston, Tex.) bycontinuous mixing in a solution containing water and either 3.0 percentnitric acid (Gel A) or 4.5% nitric acid (Gel B). The sols that resultedwere then heated to a temperature of approximately 125 degrees C. in acontinuous dryer to produce a 40% solids dispersion (Gel A) or a 42.5%solids dispersion (Gel B). The resulting maximum yield stress for Gel Awas 10,154 Pa and the resulting maximum yield stress for Gel B was80,154 Pa.

Examples 1 and 2

Shaped abrasive particles and abrasive articles of Example 1 wereprepared from Gel A. An open-faced acrylic box was used as a vacuum boxand the open face of the acrylic box was covered with a rigid plasticgrid material to allow relatively unblocked air flow into the box. Aperforated metal screen supported the plastic grid. The vacuum sourcewas a “NILFISK Advance model GMP-J-115” vacuum cleaner (Nilfisk-AdvanceInc., Plymouth, Minn.) which was capable of providing a vacuum of about6″ Hg (0.8 kPa.). The area subject to vacuum was approximately 7″×9″ (18cm×23 cm) and a vacuum gauge was attached to the acrylic box to indicatevacuum pressure within the box. A printing screen was constructed frompolycarbonate sheet that was 0.030″ (0.76 mm) thick. The aperturepattern on the printing screen was nested rows of equilateral trianglesthat were 0.110″ (2.79 mm) per side. The rows were 0.100″ (2.54 mm)apart at the apexes or bases. The apertures were made by laser machiningtechniques. A breathable release liner was used as an air permeablereceiving surface during vacuum screen printing. The breathable releaseliner was identified as “PART-WICK #4400” and was manufactured byPacoThane Technologies, Woburn, Mass.

Shaped abrasive particles for evaluation were made by first placing thebreathable release liner onto the plastic grid of the vacuum box andturning on the vacuum. The air flow causes the liner to suck down to thetop of the vacuum box and resulted in a vacuum of about 4.5″ Hg (0.6kPa) as indicated by the vacuum gage. The printing screen was thenplaced on top of the release liner. A liberal amount of Gel A was placedon top of the patterned screen and was screeded into the apertures usingan 8″ (20 cm) wide flexible steel drywall tool. The vacuum increased toabout 5.5″ Hg (0.73 kPa) after filling the printing screen apertures,indicating that the air flow through the breathable release liner haddecreased. With the vacuum still on, the printing screen was removedfrom the air permeable receiving surface leaving the surface coated withscreen printed shapes. The vacuum was turned off and the receivingsurface with the still-wet screen printed shapes was removed from thetop of the vacuum box. The receiving surface with the screen printedshapes was allowed to dry at 45 degrees Celsius for 1 hour after whichthe precursor shaped abrasive particles could be easily scraped off thereceiving surface without damaging the particles. Multiple batches ofprecursor shaped abrasive particles were made in this fashion andcollected to provide sufficient quantities for firing and subsequenttesting.

The precursor shaped abrasive particles were calcined at approximately650 degrees Celsius and then saturated with a mixed nitrate solutioncontaining 1.4% as MgO, 1.7% as Y₂O₃, 5.7% as La₂O₃, and 0.07% as CoOimpregnated at a level 70% by weight based on the weight of calcined,precursor shaped abrasive particles. Prior to impregnation, “HYDRAL COAT5” powder (0.5 micron mean particle size, available from Almatis ofPittsburgh, Pa.) was dispersed in the impregnation solution at a levelof 1.4% by weight based on the weight of the impregnation solution.Sufficient mixing was achieved to keep the “HYDRAL COAT 5” powderparticles in suspension within the impregnation solution until it wasadded to the calcined, precursor shaped abrasive particles. Once thecalcined, precursor shaped abrasive particles were impregnated, theparticles were allowed to dry after which the particles were againcalcined at 650 degrees Celsius and sintered at approximately 1400degrees Celsius to final hardness to produce shaped abrasive particles.Both the calcining and sintering were performed using rotary tube kilns.

The shaped abrasive particles were then electrostatically coated ontofiber disc backings at a level of 18 grams per disc of the shapedabrasive particles using a calcium carbonate filled make coating andcryolite filled size coating. The discs were evaluated using theGrinding Test using a 1045 hardened steel workpiece.

Shaped abrasive particles and abrasive articles of Example 2 wereprepared identically to those of Example 1 with the exception that thestiffer Gel B was substituted for Gel A. Additional shaped abrasiveparticles in Examples 4, 5, and 6 were prepared identically to those ofExamples 1 and 2 except that the gel yield stress was changed to thevalues as noted in Table 1 by varying the percent nitric acid content tointermediate levels between those used for making GEL A and GEL B.Physical parameters for the shaped abrasive particles produced by theabove method are listed in Table 1.

TABLE 1 Physical Testing Gel First Second Second Yield Side Side ScreenSide/ Thickness Stress Area Area Area First Ratio Sample Pa · Sec mm²mm² mm² Side Tc/Ti Example 1 10,154 0.3427 1.1788 3.137 3.44 1.40Example 2 80,154 0.5317 0.8750 3.137 1.65 1.28 Example 3 53,850 0.40170.8314 3.137 2.07 1.46 Example 4 29,010 0.4328 0.9616 3.137 2.22 1.37Example 5 26,366 0.3244 0.9957 3.137 3.07 1.43Grinding Test

The abrasive discs were tested using the following procedure. 7-inch(17.8 cm) diameter abrasive discs for evaluation were attached to arotary grinder fitted with a 7-inch (17.8 cm) ribbed disc pad face plate(“80514 Extra Hard Red” obtained from 3M Company, St. Paul, Minn.). Thegrinder was then activated and urged against an end face of a 0.75×0.75in (1.9×1.9 cm) pre-weighed 1045 steel bar under a load of 10 lb (4.5kg). The resulting rotational speed of the grinder under this load andagainst this workpiece was 5000 rpm. The workpiece was abraded underthese conditions for a total of thirty six (36) 20-second grindingintervals (passes). Following each 20-second interval, the workpiece wasallowed to cool to room temperature and weighed to determine the cut ofthe abrasive operation. Test results were reported as the incrementalcut for each interval and the total cut removed. If desired, the testingcan be automated using suitable equipment.

A summary of the test results are shown graphically in FIG. 5. The plotclearly shows improved performance of the shaped abrasive particles madefrom the low viscosity sol gel over the shaped abrasive particles madefrom the high viscosity sol gel. SEM photomicrographs show that the lowviscosity particles flowed or slumped significantly after the shapingprocess most likely during the drying process when compared to particlesmade using high viscosity gel. As a result, the low viscosity shapedabrasive particles have much sharper (thinner) edges resulting in higherperformance.

Other modifications and variations to the present disclosure may bepracticed by those of ordinary skill in the art, without departing fromthe spirit and scope of the present disclosure, which is moreparticularly set forth in the appended claims. It is understood thataspects of the various embodiments may be interchanged in whole or partor combined with other aspects of the various embodiments. All citedreferences, patents, or patent applications in the above application forletters patent are herein incorporated by reference in their entirety ina consistent manner. In the event of inconsistencies or contradictionsbetween portions of the incorporated references and this application,the information in the preceding description shall control. Thepreceding description, given in order to enable one of ordinary skill inthe art to practice the claimed disclosure, is not to be construed aslimiting the scope of the disclosure, which is defined by the claims andall equivalents thereto.

What is claimed is:
 1. A process for the production of shaped ceramicarticles by a screen printing process which comprises applying adispersion of a ceramic precursor to a receiving surface through aprinting screen comprising a plurality of apertures, removing theprinting screen from the receiving surface to form a plurality of screenprinted shapes while applying a differential pressure between a firstside of the screen printed shape and a second side of the screen printedshape that is in contact with the receiving surface, at least partiallydrying the screen printed shapes remaining on the receiving surface andfiring the screen printed shapes to form sintered shaped ceramicarticles.
 2. The process according to claim 1 wherein the ceramicprecursor is a chemical precursor.
 3. The process according to claim 1wherein the dispersion comprises an alpha alumina precursor.
 4. Theprocess according to claim 3 wherein the dispersion is a boehmitealumina sol gel.
 5. The process according to claim 4 wherein the sol gelhas a solids content of between about 30% to about 60% by weight.
 6. Theprocess according to claim 4 wherein the sol gel has a maximum yieldstress and the maximum yield stress is between about 60,000 Pa·Sec toabout 5,000 Pa·Sec.
 7. The process according to claim 5 wherein the solgel maximum yield stress is between about 15,000 Pa·Sec to about 5,000Pa·Sec.
 8. The process according to claim 1 wherein the applying adifferential pressure comprises applying a positive pressure to thefirst side of the screen printed shapes.
 9. The process according toclaim 8 where the applying a differential pressure comprises applying anegative pressure to the second side of the screen printed shapes. 10.The process according to claim 1 where the applying a differentialpressure comprises applying a negative pressure to the second side ofthe screen printed shapes.
 11. A process for the production of shapedabrasive particles comprising alpha alumina, the process comprisingscreen printing shapes of a boehmite alumina sol gel onto a receivingsurface to form a plurality of screen printed shapes, removing thescreen printed shapes from a printing screen while applying adifferential pressure between a first side of the screen printed shapesand a second side of the screen printed shapes that are in contact withthe receiving surface, at least partially drying the screen printedshapes and firing the screen printed shapes at a temperature sufficientto convert the alumina to the alpha phase.
 12. The process according toclaim 11 wherein the printing screen comprises apertures havingdimensions to produce screen printed shapes with an aspect ratio ofbetween about 2:1 to about 50:1.
 13. The process according to claim 11wherein the printing screen comprises a plurality of generallytriangular apertures.
 14. The process according to claim 11 wherein theapplying a differential pressure comprises applying a positive pressureto the first side of the screen printed shapes.
 15. The processaccording to claim 14 wherein a pressure transfer roll is used to applythe positive pressure.
 16. The process according to claim 14 where theapplying a differential pressure comprises applying a negative pressureto the second side of the screen printed shapes.
 17. The processaccording to claim 11 where the applying a differential pressurecomprises applying a negative pressure to the second side of the screenprinted shapes.
 18. The process according to claim 17 wherein thereceiving surface is air permeable and a vacuum box or vacuum roll ispositioned adjacent to the receiving surface opposite the second side ofthe screen printed shapes on the receiving surface.
 19. A process forthe production of shaped abrasive particles comprising alpha alumina,the process comprising screen printing shapes of a boehmite alumina solgel onto an air permeable receiving surface, the air permeable receivingsurface located adjacent to a vacuum box or vacuum roll while fillingapertures in a printing screen with the sol gel to form a plurality ofscreen printed shapes, removing the screen printed shapes from theprinting screen, at least partially drying the screen printed shapes,and firing the screen printed shapes at a temperature sufficient toconvert the alumina to the alpha phase.
 20. The process of claim 19wherein removing the screen printed shapes from the printing screencomprises applying a differential pressure between a first side of thescreen printed shapes and a second side of the screen printed shapesthat are in contact with the receiving surface.
 21. The processaccording to claim 20 wherein the applying a differential pressurecomprises applying a positive pressure to the first side of the screenprinted shapes.
 22. The process according to claim 21 wherein a pressuretransfer roll is used to apply the positive pressure.
 23. The processaccording to claim 20 where the applying a differential pressurecomprises applying a negative pressure to the second side of the screenprinted shapes.
 24. The process according to claim 23 wherein a vacuumbox or vacuum roll is positioned adjacent to the air permeable receivingsurface opposite the second side of the screen printed shapes on thereceiving surface.